1. Field of the Invention
The present invention relates to wireless charging, and more particularly, to a method for transmitting signals by a wireless power transmitter in a wireless charging system, the wireless power transmitter, and a wireless power receiver.
2. Description of the Related Art
In view of their nature, mobile terminals such as portable phones and Personal Digital Assistants (PDAs) are powered by rechargeable batteries. To charge the batteries, the mobile terminals supply electrical energy to the batteries through separate chargers. Typically, the charger and the battery each have an exterior contact terminal and thus are electrically connected to each other by contacting their contact terminals.
This contact-based charging scheme faces the problem of vulnerability of contact terminals to contamination of foreign materials and the resulting unreliable battery charging because the contact terminals protrude outward. Moreover, if the contact terminals are exposed to moisture, the batteries may not be charged properly.
To address the above problem, wireless charging or contactless charging technologies have recently been developed and applied to many electronic devices.
Such a wireless charging technology is based on wireless power transmission and reception. For example, once a portable phone is placed on a charging pad without being connected to an additional charging connector, its battery is automatically charged. Among wirelessly charged products, wireless electric toothbrushes or wireless electric shavers are well known. The wireless charging technology offers the benefits of increased waterproofness due to wireless charging of electronic products and enhanced portability due to no need for a wired charger for electronic devices. Further, it is expected that various relevant wireless charging technologies will be further developed in the upcoming era of electric vehicles.
There are mainly three wireless charging schemes: an electromagnetic induction scheme using coils, a resonance-based scheme, and a Radio Frequency (RF)/microwave radiation scheme based on the conversion of electrical energy to microwaves.
To date, the electromagnetic induction-based wireless charging scheme is most popular. However, considering recent successful experiments in wireless power transmission over microwaves at a distance of tens of meters in Korea and in other overseas countries, it is foreseeable that every electronic product will be charged wirelessly at any time in any place in the near future.
Electromagnetic induction-based power transmission refers to power transfer between primary and secondary coils. When a magnet moves around a coil, a current is induced. Based on this principle, a transmitter generates a magnetic field and a receiver produces energy by a current induced by a change in the magnetic field. This phenomenon is called magnetic induction and power transmission based on magnetic induction is highly efficient in energy transfer.
Regarding resonance-based wireless charging, a system was suggested for wireless energy transfer from a charger at a distance of a few meters based on the resonance-based power transmission principle by the coupled mode theory. Electromagnetic waves were resonated to carry electrical energy, instead of sound. The resonant electrical energy is directly transferred only in the presence of a device having the same resonant frequency, while the unused electrical energy is reabsorbed into the electromagnetic field rather being dispersed in the air. Thus, the resonant electrical energy does not affect nearby machines or human beings, as compared to other electrical waves.
A method in which a wireless power transmitter (or a Power Transmitting Unit (PTU)) detects or determines whether a wireless power receiver (or Power Receiving Unit (PRU)) is placed thereon may include a method of detecting a change in impedance of a power transmission unit included in the wireless power transmitter.
If a wireless power transmitter (PTU) detects the presence of a wireless power receiver (PRU) through an impedance change detection and the like, the wireless power transmitter may initiate communication with the wireless power receiver by supplying the power with which the wireless power receiver can perform communication.
For example, in accordance with the wireless power standard of Alliance for Wireless Power (A4WP), a wireless power transmitter may transmit a long-beacon signal, and upon receiving the long-beacon signal, a wireless power receiver may transmit an advertisement signal to the wireless power transmitter within a predetermined time, thereby proceeding with a registration procedure for wireless charging.
However, where a boot procedure for operating a processor is required while the battery power of the wireless power receiver is low or depleted or the wireless power receiver is powered off, the wireless power receiver may not transmit the advertisement signal to the wireless power transmitter within a predetermined time. If the wireless power receiver cannot transmit the advertisement signal within a predetermined time in this way, the normal registration procedure may not be performed, so wireless charging for the wireless power receiver may not be possible.